1. Field of the Invention
The present invention relates to systems and methods for determining the state of health of a battery, as well as batteries and related devices that facilitate determining the state of health. In particular, the present invention is related to a system and method for determining the state of health of an electrochemical battery cell by obtaining state of charge measurements for the battery cell, and using the state of charge measurements to calculate the state of health of the battery cell, wherein the state of charge measurements are based on the magnetic susceptibility of the battery cell.
2. Description of the Related Art
A battery includes one or more battery cells, connected in a series and/or parallel arrangement, that chemically store electrical charge potential (energy) and deliver the charge at a pre-determined voltage when demanded by an external electric circuit load. Each of the battery cells contains two half-cells connected in series by an electrolyte, which may be a solid or a liquid. The electrolyte consists of anions (i.e., negatively-charged ions) and cations (i.e., positively-charged ions). One of the half-cells contains some of the electrolyte and an anode (i.e., negative electrode), toward which anions migrate. The other half-cell contains some of the electrolyte and a cathode (i.e., positive electrode) toward which cations migrate. The electrodes do not touch each other but are electrically connected by the electrolyte.
During battery cell operation, a redox (reduction-oxidation) reaction powers the battery cell. That is, the cations in the electrolyte are reduced (i.e., by the addition of electrons) at the cathode, and the anions are oxidized (i.e., by the removal of electrons) at the anode. As a battery cell discharges, ions flow from the anode, through the electrolyte, to the cathode. As the battery cell charges, the ions flow from the cathode, through the electrolyte, to the anode.
The state of charge of a battery cell refers to the instant charge of the battery cell relative to the charge of the battery cell when the battery cell is fully charged. For example, the state of charge of a battery cell may decrease from approximately 100% to approximately 0% during a complete discharge cycle, and may increase from approximately 0% to approximately 100% during a complete charge cycle.
A theoretically perfect battery cell is capable of storing a charge that is a function of its design parameters and materials, delivering the charge to an external electrical load, and then being fully recharged to its original capacity. Thus, if one were to measure the total charge (i.e., amp-hours) entering the battery cell during a charge cycle, and subtract from that measurement the total charge exiting the battery cell during a discharge cycle, the resulting value would be an accurate indicator of the state of charge of the battery cell (i.e., the amount of energy stored within the battery cell).
Because the charge is stored chemically, however, each charge/discharge cycle (as well as normal temperature cycling, vibration, shock, etc.) results in irreversible changes within the individual battery cells, the changes affecting battery capacity. Moreover, the rate of charge and/or discharge can also manifest in changes to battery capacity. The common result of these changes is that less energy is stored in the battery during each subsequent charge cycle. The state of health of a battery cell refers to the instant capacity of the battery cell relative to the original capacity of the battery cell. For example, the state of health of a battery cell may be approximately 100% upon manufacturing of the battery cell, and may continually decrease as the age and/or use of the battery cell increases.
Because the state of health of a battery cell degrades over time, the theoretical state of charge, which can be determined by the aforementioned method of subtracting the amount of charge used from the amount of charge initially placed in the battery cell, may differ from the actual state of charge of the battery cell (i.e., the theoretical state of charge will be greater than the actual state of charge due to battery cell degradation).
Accordingly, there is a need for a system and method for determining the state of health of a battery cell, as the state of health provides an indication of the capacity of the battery cell, the age of the battery cell, and the rate at which the battery cell ages.
U.S. Pat. No. 6,668,247 describes a system and method for determining the state of health of a lead-acid battery, the system and method involving monitoring impedance of the battery and training a fuzzy system in a relationship between the impedance and the number of charge cycles the cell has undergone. U.S. Pat. No. 7,072,871 describes a method for determining the state of health of a battery, which may have one of a variety of chemistry types, by measuring multiple parameters, such as internal resistance, voltage, temperature, etc., and training a fuzzy system to provide the battery state of health. Those prior art methods of assessing the state of health, however, are complicated and depend on many battery parameters and extensive records of historical data related to the batteries.